This invention relates primarily to sounds generated by the beating heart in the audible range and their use for adaptive pacing and has particular application to optimized pacing for congestive heart failure including but not limited to conditions known as hypertrophic and dilated cardiomyopathy as well as other conditions associated with turbulent or other aberrant blood flow conditions in the heart. Its teaching may be applied to hemodynamic control generally.
Over the centuries heart sounds have been used to monitor heart function by practitioners of medicine. Even today when one visits a doctor's office, almost invariably monitoring one's heart sounds through a stethoscope is conducted by examining physicians.
Nevertheless, these direct sound signals generated by the heart have not been used previously to optimize the timing of pacing pulses delivered to a paced heart. Because the conditions of hypertrophic and dilated cardiomyopathy may be associated with acoustical range noises made by the heart related to mitral valvular regurgitation it can be demonstrated that this indication provided by these sounds can be put to use. The direct employment of this diagnostic parameter to the pacing of the heart has not, however, been previously employed nor has any apparatus or system been developed to take advantage of this naturally occurring indicator.
In order to take advantage of this useful feature, the invention here adjusts the pacemaker AV delay (which may also be called "AV interval" or "AV escape interval") by reference to the sound signals generated by the heart. The literature supports the concept of adjustment of the AV delay to optimize pacing particularly in Hypertrophic Obstructive Cardiomyopathy (HOCM) patients. This invention bases the AV interval timing or adjustment on sound and shows how it can be applied to HOCM and other congestive heart patients.
For a discussion on how to adjust the electrocardiographic PQ interval by setting the AV delay with a DDD pacemaker, see "Critical PQ Interval for the Appearance of Diastolic Mitral Regurgitation and Optimal PQ Interval in Patients Implanted with DDD Pacemakers" by Ishikawa et al., PACE, Vol. 17, November, 1994, Part II.
Two United States patents also illustrate adaptively changing the AV interval (or "PV" interval in one case) both to avoid fusion with the natural ventricular depolarization of a patient's heart and to assist patients suffering from cardiomyopathy to improve cardiac output. See Sholder, U.S. Pat. Nos. 5,340,361 and 5,334,220.
The value of dual chamber cardiac pacing in treatment of patients suffering from HOCM has been recognized and the benefits of this therapy are discussed in the articles "Permanent Pacing as Treatment for Hypertrophic Cardiomyopathy" by Kenneth M. McDonald et al., published in the American Journal of Cardiology, Vol. 68, Jul. 1, 1991, pp. 108-110, "Impact of Dual Chamber Permanent Pacing in Patients with Obstructive Hypertrophic Cardiomyopathy with Symptoms Refractory to Verapamil and .beta.-Adrenergic Blocker Therapy" by Fananapazir et al., published in Circulation, Vol. 8, No. 6, June, 1992, pp. 2149-2161, "Effects of Dual-Chamber pacing in Hypertrophic Obstructive Cardiomyopathy", by Jeanrenaud, et al., published in The Lancet, Vol. 33, May 30, 1992, pp. 1318-1323, "Altered Cardiac Hemodynamic and Electrical State in Normal Sinus Rhythm After Chronic Dual-Chamber Pacing for Relief of Left Ventricular Outflow Obstruction in Hypertrophic Cardiomyopathy", by McAreavey et al., published in American Journal of Cardiology, 1992, Vol. 70, pp. 651-656, and "Effects of Dual-Chamber Pacing in Hypertrophic Cardiomyopathy Without Obstruction", by Seidelin et al., published in The Lancet, 1992, pp. 340-369. In these papers, the value of DDD pacing employing a shortened A-V escape interval is discussed. In particular, the use of an A-V escape interval which is shorter than the patient's intrinsic A-V conduction is specifically recommended, with favorable results being reported so long as the duration of the pacemaker's A-V escape interval is not so short that hemodynamic performance is compromised. Various approaches to selecting the optimal A-V escape interval are discussed in the literature, but none are based on heart sounds. In the cited studies, the favored signal to use for adjustment of the AV delay is a processed Doppler signal, which in the current state of the art cannot easily be done with an unassisted implantable device. Physicians have in the past used Doppler measurements of the subaortic pressure gradient to adjust the AV interval. Doppler measurements require significant processing time and power to yield useful data signals and additional sensing and output hardware. If direct sound volume signals are used the device can be much simpler. See, "Effects of Dual Chamber Pacing with Short Atrioventricular Delay in Dialated Cardiomyopathy", Brecker et al., The Lancet, Vol. 340, Nov. 28, 1992, pp. 1308-12 for an example of current usage of phonocardiogram versus Doppler, (esp. FIG. 1).
In making AV delay adjustments, pre-excitation of the ventricular apex and septum by the ventricular pacing pulse prior to excitation due to natural conduction between the atrium and ventricle is generally preferred. The abstract "How to Optimize Pacing Therapy in Patients with Hypertrophic Obstructive Cardiomyopathy: The Importance of AV Delay Programming" by Gras, et al., published in PACE, May, 1993, Vol. 16, Part II, page 1121 suggests that the longest A-V escape interval which provides complete ventricular capture should be selected. The above-cited article by Fananapazir suggests that the A-V escape interval which allows for maximal pre-excitation of the ventricle by the pacing pulse can be selected by employing the A-V escape interval that produces the widest paced QRS complex duration. The above-cited McDonald article suggests that the A-V escape interval should be set at the longest duration that maintains ventricular capture at maximum exercise levels.
In the abstract "The Optimal Patient for Pacemaker Treatment of Hypertrophic Obstructive Cardiomyopathy (HOCM)" by Jeanrenaud et al, published in PACE, May, 1993, Vol. 16, Part II, page 1120, it is suggested that in the case of patients who would require an excessively short A-V escape interval in order to accomplish pre-excitation, intrinsic A-V conduction time could be prolonged by means of drugs or ablation techniques.
We believe these references establish that there is an optimal range of AV delay to produce maximum hemodynamic benefits through pacing. This optimal AV delay is also expected to be associated with a minimum degree of mitral valvular regurgitation. Other studies have established that the curve of pulmonic artery pressure (a measure of hemodynamic function) versus AV delay has a theoretical local minimum below the range of optimal AV delay. Accordingly, in adjusting the AV delay, one must be careful not to overlengthen or overshorten it if there are to be benefits to the therapy.